MM - 60506 - notify user failed scheduled msgs (#29208)

* MM-60506 - notify user for failed scheduled messages

* add unit tests for handleFailedScheduledMessages and send system-bot message

* fix vet issues

* adjust test for vet report

* make sure to send the message to every user there was a failed message

---------

Co-authored-by: Harshil Sharma <harshil.sharma@mattermost.com>

server/i18n/en.json

@@ -6542,6 +6542,13 @@
     "id": "app.save_scheduled_post.save.app_error",
     "translation": "Error occurred saving the scheduled post."
   },
+  {
+    "id": "app.scheduled_post.failed_messages",
+    "translation": {
+      "one": "Failed to send {{.Count}} scheduled post.",
+      "other": "Failed to send {{.Count}} scheduled posts."
+    }
+  },
   {
     "id": "app.scheme.delete.app_error",
     "translation": "Unable to delete this scheme."